1. Field of the Invention
This invention relates to a lateral mode control type of AlGaInP semiconductor laser capable of being used for light communication, optical erasure, recording and reproduction of information, and the like.
2. Description of the Related Art
Semiconductor lasers have gained in importance as a light source for information processing units using optical disks or the like. In particular, AlGaInP semiconductor lasers of a 600 nm wavelength band have attracted attention, and the development of such lasers is being promoted.
Conventional AlGaInP semiconductor lasers have a structure such as that illustrated in section in FIG. 3. Such a device structure is ordinarily formed by a process described below in accordance with a crystal growth method such as a metal organic vapor phase epitaxy (MOVPE) method.
In a first crystal growth step, a Se-doped n-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P clad layer 2, an undoped Ga.sub.0.5 In.sub.0.5 P active layer 3, a Zn-doped p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P clad layer 40, a Zn-doped p-GaAs cap layer (not shown) are successively laminated on an n-GaAs substrate 1.
Next, the Zn-doped p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P clad layer 40 is etched by using an SiO.sub.2 mask (not shown) so that an unetched portion has a trapezoidal cross section, thereby forming a mesa stripe in the &lt;110&gt; direction. Thereafter, in a second crystal growth step, an n-GaAs current stricture layer 5 is embedded in side surfaces of the mesa stripe by being selectively grown.
Then, the SiO.sub.2 mask (not shown) is removed and, in a third crystal growth step, a p-GaAs contact layer 6 is laminated over the entire surface. Finally, a p side electrode 7 and an n side electrode 8 are formed.
In a semiconductor laser having this device structure, current-stricture can be effected by the n-GaAs current stricture layer 5. Also, laser light is guided by being confined under the mesa stripe. A lateral waveguide structure is formed in this manner.
The above-described conventional device structure, however, entails problems described below.
A phenomenon is inevitable that impurities, such as Se and Zn added in n- and p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P clad layers 2 and 40, diffuse in a crystal during crystal growth or during a heat treatment. The influence of this phenomenon is serious particularly in a case where a plurality of crystal growth steps such as those mentioned above are required. It is thereby possible that a PN junction position is changed and impurities diffused in undoped Ga.sub.0.5 In.sub.0.5 P active layer 3 generate non-emissive recombination centers in the active layer, thereby deteriorating device characteristics.
In a material such as an AlGaInP material forming a crystal in which an orderly structure called a natural super lattice is formed, disordering of the natural super lattice is promoted as impurities such as Se and Zn are diffused, and a band gap of the crystal is changed according to the extent of diffusion. That is, impurities added in n- and p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P clad layers 2 and 40 are diffused in undoped Ga.sub.0.5 In.sub.0.5 P active layer 3 to change a band gap thereof. This effect is increased if the impurity concentration is higher. This phenomenon has restricted the improvement in the oscillation wavelength reproducibility of semiconductor lasers.